1. Field of the Invention
The present invention generally relates to perforating apparatus, and, more particularly, to orienting apparatus for use in a perforating gun string.
2. Description of the Prior Art
For purposes of enhancing production from a subterranean formation, a perforating gun typically is lowered down into a wellbore that extends through the formation. A perforating gun may, for example, comprise a plurality of radially-oriented shaped charges which are detonated to form perforations in the formation proximate the wellbore. The shaped charges may, for example, be placed at points along a helical spiral that extends around a longitudinal axis of the perforating gun.
In certain applications of perforating guns, orientation apparatus is included in the perforating gun string to orient the string correctly. Orientation apparatus is, for example, commonly used in highly deviated or horizontal wells. The purpose of the orientation apparatus is to provide torque for correct orientation of the perforating string.
An internal swiveled gun is one type of orientation apparatus that has been utilized. In each perforating gun, the two ends of its loading tube are mounted with bearings. By special arrangement of the charges loaded in the tube and gravitational force, the correct orientation of the perforating gun can be realized. The drawbacks of such a design are it is expensive, it lacks reliability and it is difficult to operate.
Weight spacer assembly (WSA) is another way to realize the oriented perforating. WSA is an independent subassembly used together with a gun string and comprises a hollow carrier, a spacer tube having a weight tray and weight bars. The tray is made from a half round tube and partitioned into a small weight storage tray. Weight bars are placed inside the weight trays and permanently fastened inside the tray. Therefore, weight bars and the tray have to be handled as a whole at all times. The weight of the weight bar and tube is substantial with a 10 foot 7.00″ WSA weight bar weighing up to 800 lbs. Handling of the weight spacer assembly thus imposes huge difficulties for manufacturing and operation (in assembling and disassembling).
For example, a crane or other lifting equipment must be used to effect the insertion of the spacer tube into the carrier. Because of the substantial weight of a loaded spacer tube, the spacer tube is subject to being bent or severely deformed during the handling. Yet another issue with this heavy loading tube arises once the spacer tube is successfully placed inside the carrier. Keys of the alignment plates in the spacer tube need to be placed in the keyways of the carrier. Because of the tight tolerances between the alignment plate, the carrier and the massive weight, it is an arduous task for the operators to complete this assembling step.